High-voltage power transmission cables have a center conductor surrounded by a semi-conducting shield between the conductor and the insulation of the cable. A semi-conducting shield surrounds the insulation, and there is an outer shield, usually a corrugated metal shield, around the semi-conducting shield of the insulation.
Such transmission lines are subjected to operating conditions not met with in other types of cables. It is important to prevent water or water vapor from gaining access to the insulation. Polyethylene insulation, which is the insulation most commonly used on such cables, is damaged by the formation of electro-chemical trees when moisture is present at regions of high electrical stress in the insulation. The voltages used for high-voltage power transmission lines are such as to cause regions of high electrical stress in the insulation.
Power transmission lines carry much heavier loads when demand is high; and the loads are greatly reduced at other times. This variation in the amount of current flowing through the conductors causes substantial variations in temperature at different times.
This "load cycling" results in expansions and contractions of the cable. When successive layers of the cable construction do not maintain their adhesion to one another, the expansion and contraction of the cable causes cracking at the interfaces of the layers and results in voids in which moisture can accumulate, and travel lengthwise of the cable where the voids intersect one another and provide longitudinal clearance between layers of the cable structure.
It has been the practice to use filler compounds within high voltage power transmission cables to prevent formation of voids and migration of moisture lengthwise of the cable between successive cable layers.
Such fillers or flooding compounds may be satisfactory on new cables; but as the cable ages, the compound may lose its adhesive qualities or may harden and fracture and portions of it may migrate into other layers of the cable, particularly the insulation layer. Such migration may facilitate the formation of electrochemical trees and greatly shorten the useful life of the cable.
There are many fillers and flooding compounds available commercially. I have tested the most promising of these compounds in extensive cable research tests and could not find any compound that had and retained all of the desired characteristics for a high-voltage transmission cable over long periods of time when subject to load cycling; high temperatures encountered under overload conditions; exposure to moisture, particularly at elevated temperatures and aging that tends to reduce adhesiveness and flexibility.
This invention is based on the discovery that certain compounds of isobutylene rubber do provide all of the desirable filler characteristics for a high-voltage power transmission cable. Specific compounds and their correlation in the cable construction will be explained more fully as the description proceeds.